The present invention relates to mobile communications system equipment having multiple antenna panels that are used for Multiple Input Multiple Output (MIMO) transmissions, and more particularly to technology for configuring spatially Quasi Co-Located (QCL) reference signal resources for use by the mobile communications system equipment when performing transmissions, such as codebook based MIMO transmissions.
The standardized organization of communications networks, as well as the designs of individual network elements and other equipment that form and/or interact with the network, continue to evolve in response to ever increasing demands for higher performance and capacity in mobile communications systems. One aspect of this evolution involves the use of the electromagnetic spectrum in bands located at higher frequencies than have been used in earlier generation equipment. This use means, in turn, that narrow beam transmission and reception schemes will be needed at higher frequencies to compensate for the high propagation loss between the User Equipment (UE) and the networks' Transmission/Reception Point (TRP). As used in this specification, the term UE can refer to any wireless communication device that is directly operated by an end user such as, but not limited to, the following examples: cellular or other wireless telephones, personal digital assistants, tablets and other personal computing devices equipped with wireless communication equipment, machine type communication devices, and the like. Further, as used throughout this specification, the term TRP can refer to any radio communication equipment such as, but not limited to, the following examples: Base Transceiver Stations (BTS), Base Station Controllers (BSC), relay nodes (RN), Remote Radio Heads (RRH), NodeB, eNodeB, gNodeB (gNB), and the like, such being defined by the various communications standards promulgated by standardization bodies (e.g., Third Generation Partnership Project—3GPP), such as Global System for Mobile Communication (GSM), Universal Mobile Telecommunications Service (UMTS), Long Term Evolution (LTE), and most recently, New Radio (NR). Such network equipment may be referred to herein as “nodes”. Historically, such nodes have been implemented as processing equipment configured in one location. More recently, the functionality of a single node may in some, but not necessarily all, instances be distributed among a plurality of processing elements that are distributed within the communications network, and which interact with one another in a seamless way such that any device interacting with such a virtual node has no way of knowing whether the functionality is being provided by a single processing equipment (herein also referred to as “element”) or by a plurality of communication network elements. To facilitate the discussion, this description will refer to communications between a UE and a network node. However, it will be understood that the term “network node” refers to any type of TRP that is capable of carrying out the described functionality, regardless of implementation (e.g., the term “network node” can refer to one or more communication network elements cooperating within the network to accomplish functions attributed to the “node”).
For a given communication link, beams can be applied at the network node and also at the UE (one transmitting, the other receiving), which will herein be referred to as a beam pair link (BPL). A beam management procedure is performed, whose task is to establish and maintain beam pair links To illustrate this point, FIG. 1 depicts a network node 101, a UE 103, and a BPL 105 that connects them. In order to establish the BPL 105, the network node may have tried any of the candidate beams 107, before settling on a best one for use in the BPL 105. The network thereafter maintains the BPL 105 for further communication between the UE 103 and network node 101. Both the transmit and receive beams of the BPL 105 are established and monitored by the network using measurements on downlink reference signals used for beam management. For example, it has been agreed by the 3GPP in its standardization of NR, that Channel State Information-Reference Signals (CSI-RS) will be the beam reference signals. The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodically (event triggered), and they can be either shared between multiple UEs or be UE-specific. In order to find a suitable network node beam, the network node transmits CSI-RS in different network node transmission (TX) beams on which the UE performs Reference Signal Received Power (RSRP) measurements, and reports back some number (N) of the best node TX beams (where N can be configured by the network). Furthermore, the CSI-RS transmission on a given node beam can be repeated to allow the UE to evaluate suitable UE beams (i.e., UE reception—RX—beam training).
There are primarily three different implementations of beamforming, both at the network node and at the UE: analog beamforming, digital beamforming and hybrid beamforming. Each implementation has its pros and cons. Digital beamforming is the most flexible solution but also the costliest due to the large number of required radios and baseband chains. Analog beamforming is the least flexible but the cheapest to manufacture due to reduced number of required radio and baseband chains. Hybrid beamforming is a compromise between the analog and digital beamforming implementations.
One type of beamforming antenna architecture that has been agreed to study in 3GPP for the NR access technology involves the use of antenna panels, both at the network node side and at the UE. A panel is an antenna array of single- or dual-polarized elements with typically one transmit/receive unit (TXRU) per polarization. An analog distribution network with phase shifters is used to steer the beam of each panel. FIGS. 2A and 2B illustrate two examples of dual-polarized panels, with FIG. 2A illustrating a two-dimensional panel 201, and FIG. 2B illustrating a one-dimensional panel. The two-dimensional panel 201 has a pair of connection points 205 for connection to one TXRU (not illustrated), one connection point per polarization. The one-dimensional panel 203 is similarly configured with a pair of connection points 207.
Uplink Beam Management
Some UEs might have analog beamformers without beam correspondence, which means that Downlink/Uplink (DL/UL) reciprocity cannot be used to determine the beams for these beamformers. For such UEs, the UE beam used for UL cannot be derived from beam management procedures based on DL reference signals as described above. To handle such UEs, UL beam management has been included in the NR standard specification for release 19. The main difference between normal beam management and UL beam management is that UL beam management utilizes uplink reference signals instead of DL references signals. The UL reference signals that have been agreed to be used for UL beam management are Sounding Reference Signals (SRS). Two UL beam management procedures, called U2 and U3, have been discussed during the standardization of NR. These are schematically illustrated in FIGS. 3A and 3B, respectively. Looking first at FIG. 3A, the U2 procedure is performed by transmitting a burst of SRS resources in one UE TX beam 301 and letting the network node 303 evaluate different TRP RX beams 305. And as illustrated in FIG. 3B, the U3 procedure lets the network node 303 select a suitable (“best”) UE TX beam by having the UE 307 transmit different SRS resources in different UE TX beams 309, and then assessing the received transmissions based on one or more predefined transmission selection criteria (e.g., comparing the different beams using any of the measurements of received signal quality that are known in the art).
It will be understood that, as used herein, the term “SRS resource” refers to a configuration of a number of parameters that control how one or more SRSs are transmitted, and is exemplified by SRS resources as defined and discussed in, for example, Section 6.2.1 “UE sounding procedure” of the specification, 3GPP TS 38.214 V15.0.0 (2017-12), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NR; Physical layer procedures for data (Release 15)”, December 2017.
Codebook Based UL Transmission
In addition to their use for UL beam management as described above, SRS resources are also used to help normal UL transmissions, for example when performing a so-called Codebook-based UL transmission, which has been standardized in NR. Codebook based UL transmission relies on a multi-antenna configuration to support uplink MIMO communications with up to 4 layer spatial multiplexing using up to 4 antenna ports with channel dependent precoding. The spatial multiplexing mode aims for high data rates in favorable channel conditions.
FIG. 4 is an exemplary embodiment of an arrangement 400 for performing precoded spatial multiplexing when Cyclic Prefix-Orthogonal Frequency Division Multiplexing (CP-OFDM) is used on the uplink As seen in the figure, information to be transmitted partitioned into a number, r, separate Layers, where the number r is called the “transmission rank.” Each Layer 401-x supplies one symbol to a respective one of r inputs of the precoder matrix W, forming an information-carrying symbol vector, s. The symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT-dimensional vector space (corresponding to NT antenna ports). The precoder matrix W is typically selected from a codebook of possible precoder matrices, with selection typically being indicated by means of a transmit precoder matrix indicator (TPMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams The NT weighted symbols supplied at the output of the precoder matrix W are supplied to respective ones of NT Inverse Fast Fourier Transform (IFFT) processors 403-x. The outputs of the IFFT processors 403-x are supplied to respective ones of NT antenna ports. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
The received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) (where NR is the number of receiver antennas) is thus modeled byyn=HnWsn+en  Equation 1where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix W is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
One example method for a UE to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
                              max          k                ⁢                                                                                          H                  ^                                n                            ⁢                              W                k                                                          F          2                                    Equation        ⁢                                  ⁢        2            Where                Ĥn is a channel estimate, possibly derived from CSI-RS.        Wk is a hypothesized precoder matrix with index k.        ĤnWk is the hypothesized equivalent channel.        
In closed-loop precoding for the NR uplink, the network node decides, based on channel measurements in the reverse link (uplink), what TPMI the UE should use on its uplink antennas, and transmits this TPMI to the UE. The gNodeB configures the UE to transmit the SRS according to the number of UE antennas it would like the UE to use for uplink transmission, in order to enable the channel measurements. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be signaled. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, for example, several precoders and/or several TPMIs, one per subband.
Information other than the TPMI is generally used to determine the UL MIMO transmission state, such as SRS resource indicators (SRIs) as well as transmission rank indicators (TRIs). These parameters, as well as the modulation and coding scheme (MCS), and the uplink resources where the Physical Uplink Shared Channel (PUSCH) is to be transmitted, are also determined by channel measurements derived from SRS transmissions from the UE. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder matrix, W. For efficient performance, it is important that a transmission rank that matches the channel properties be selected.
SRS Resource Set
The network node needs to signal to the UE various parameters that control how the SRS transmission should be done. Such parameters include, for example, which SRS resource to use, the number of ports per SRS resource, and the like. This is solved in NR by defining a number of SRS resource sets using higher layer signaling (e.g., Radio Resource Control—RRC)—and/or Medium Access Control-Control Element—MAC-CE), where each SRS resource set contains a list of different SRS resources. For NR release 15, each UE can be configured to have a number of different SRS resource sets, including:                one SRS resource set for codebook based UL transmission, and        multiple SRS resource sets for UL beam management.        
The different SRS resources within an SRS resource set can have different time domain behavior. For example in a SRS resource set consisting of four SRS resources, two SRS resources can be configured with periodic time domain behavior, while the other two can be configured with aperiodic time domain behavior. The periodic SRS resources in an SRS resource set are triggered by using RRC signaling, the SRS resources with semi-persistent time domain behavior are triggered by using Medium Access Control/Control Element (MAC/CE) signaling, and the aperiodic SRS resources are triggered by using DCI signaling.
In case the UE is equipped with one or more analog beamformers, the SRS resource sets can be configured with a spatial QCL relation to indicate to the UE which analog UE beam (i.e., BPL) to use during the SRS transmission. The spatial QCL relation is configured using the higher layer parameter SRS-SpatialRelationInfo which can be defined for each SRS resource set. (Multi-layer communications protocols such as The Open Systems Interconnection model—OSI model—are well-known, and as used herein, the term “higher layer” means any layer higher than Layer 1, the Physical Layer.) The SRS-SpatialRelationInfo can point to a DL reference signal such as SSB/PBCH or CSI-RS (in case of beam correspondence) or to UL reference signals such as SRS (in case of no beam correspondence). So, for example, a UE without beam correspondence can first perform a U3 procedure by transmitting different SRS resources in different UE TX beams The network node measures RSRP of the different SRS resources and determines which SRS resource gives the highest RSRP. The network node can then use higher layer signaling to update the SRS-SpatialRelationInfo (for a given SRS resource set) with the best SRS resource. After this update, the next time the UE is triggered for SRS transmission for that SRS resource set, the UE will know which analog UE TX beam to apply when transmitting the SRS resources.
One example for periodic SRS transmissions, is published in the earlier-mentioned specification, 3GPP TS 38.214 V15.0.0 (2017-12), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NR; Physical layer procedures for data (Release 15)”, December 2017:                For a UE configured with one or more SRS resource configuration(s), and when the higher layer parameter SRS-ResourceConfigType is set to ‘periodic’: p2 if the UE is configured with the higher layer parameter SRS-SpatialRelationInfo set to ‘SSB/PBCH’, the UE shall transmit the SRS resource with the same spatial domain transmission filter used for the reception of the SSB/PBCH, if the higher layer parameter SRS-SpatialRelationInfo is set to ‘CSI-RS’, the UE shall transmit the SRS resource with the same spatial domain transmission filter used for the reception of the periodic CSI-RS or of the semi-persistent CSI-RS, if the higher layer parameter SRS-SpatialRelationInfo is set to ‘SRS’, the UE shall transmit the SRS resource with the same spatial domain transmission filter used for the transmission of the periodic SRS.UL Beam Management for Multi-panel UEs        
It is expected that the UE will use two or more antenna panels, preferably pointing in different directions, in order to improve the coverage and increase the order of spatial multiplexing. FIG. 5 illustrates a non-limiting example of a UE 501 having two one-dimensional antenna panels 503, 505 located in different directions. In order to handle UL beam management for such UEs in an efficient manner (to minimize overhead), it has been agreed in the NR standard that the network node can trigger the UE 501 to transmit one SRS resource set per UE antenna panel, where each SRS resource set consists of a number of SRS resources (corresponding to the number of candidate beams per UE antenna panel 503, 505). When so triggered, the UE 501 transmits one SRS resource per beam per panel while the network node performs RSRP measurements on the SRS resources. The network node assesses these measurements and determines the best SRS resource per SRS resource set and in that way the network node can determine the best UE TX beam per panel.
The inventor of the embodiments described herein has recognized that the existing technology suffers from one or more problems. For example, conventional technology enables a network node to use higher layer signaling to cause a UE to configure the SRS-SpatialRelationInfo parameter for only a single antenna panel. As a result, a UE having more than one antenna panel but lacking beam correspondence (i.e., having no ability to use DL/UL reciprocity to derive a suitable beam for UL transmissions based on DL reference signals) would still not be able to benefit by the improved performance that would otherwise be achievable if it could know which beam to use for more than one of its antenna panels.
Hence, there is a need for technology that addresses the above and/or related issues.